1. Field of the Invention
The present invention relates to a lighting controller for a lighting device for a vehicle, and more particularly to a lighting controller for a lighting device for a vehicle constructed so as to control the lighting of a semiconductor light source composed of semiconductor light emitting element.
2. Background Art
As a lighting device for a vehicle, a lighting device using a semiconductor light emitting element such as an LED (light Emitting Diode) as a light source has been hitherto known. On such kind of lighting device for a vehicle, a lighting control circuit for controlling the lighting of the LED is mounted.
When the LED is controlled to be turned on using the lighting control circuit, a control for always supplying a constant current to the LED can be employed. However, the LED has characteristics that when the temperature of the LED rises, even if the same forward current is supplied to the LED, the light flux of emitted light (a quantity of light) is lowered. Therefore, when a control for always supplying a constant current to the LED is carried out, the quantity of light is sequentially lowered due to the self-heat generation of the LED itself. Especially, when a heat radiating structure is used for the purpose of raising the temperature to a temperature as high as the maximum junction temperature of the LED in view of cost or size, the amount of decrease of the quantity of light is more drastic. When the quantity of light is lowered, a visibility is lowered for a driver, so that there is a fear that the driver cannot perform driving safely. Further, when the LED is used as a headlight or a signal light of a vehicle, the LED may possibly not satisfy required product standards.
Further, because the LED has an unevenness of the forward voltage Vf, when a prescribed current is supplied to the LED, an electric power applied to the LED having a higher forward voltage Vf is high, so that a heat generation is greater. Accordingly, as the lighting control circuit, a lighting control circuit needs to be used that has a capability and a size that allows supply of the electric power anticipating the unevenness of the forward voltage Vf of the LED. Further, the heat radiating structure of the LED needs to have a size, a form, and a thermal resistance anticipating the heat generation of the LED.
Thus, to ensure a necessary quantity of light even when the temperature of the LED rises, a lighting control circuit has been proposed in which a time during which the LED continuously emits light is measured and a current supplied to the LED is increased in accordance with the measured time (see Patent Document 1).
[Patent Document 1] JP-A-2004-330819.
As described in Patent Document 1, the lighting time during which the LED continuously emits light is measured and the current supplied to the LED is controlled to increase in accordance with the measured time, so that the quantity of emitted light of the LED can be prevented from falling in accordance with the rise of temperature.